An extruder normally comprises one or more screws driven in rotation inside an elongated sleeve in which cylindrical bores are formed for housing the screws. The latter are provided on their periphery with screw threads which drive toward the downstream end of the sleeve the material which is introduced through the upstream end of the sleeve. By acting on the pitches of the screws, the material may be subjected to trituration, compression and kneading effects, for example so as to produce polymerization in the case of granules of plastic material. The material is thus driven to the downstream end of the sleeve, which may be provided with an extrusion die or a simple outlet orifice in the case where the material does not have to be subjected to an extrusion.
In order to vary the processing achieved in the course of the advance of the material through the sleeve, the screws are formed with successive zones having threads of different pitches and shapes. For example, there may be employed conveying sections having a large pitch at the entrance of the machine so as to achieve a rapid driving of the material toward the downstream end, or along the sleeve so as to achieve an expansion or a de-gasing of the material. In other sections, the threads will have a smaller pitch or even a reverse pitch so as to retard and consequently compress the material. The threads having a reverse pitch may be provided with an opening controlling the passage of a certain amount of material toward the downstream end and increasing the trituration effect.
It is often necessary to modify the arrangement of the sections having different pitches, for example to change or adapt the processing procedure. Further, the threads may be deteriorated or subjected to a considerable wear, for example in the case where the material conveyed is particularly abrasive. This wear is obviously greater in the most compressed zones.
It is consequently necessary to be in a position to change or replace some sections of the screws and this is why these are usually formed by hollow juxtaposed elements mounted on a central shaft connected to a motor driving the shaft in rotation. In order to ensure that the rotation of the shaft is transmitted to the screw elements, the latter are provided on their inner wall with grooves which engage corresponding splines formed along the central shaft. In the course of rotation, the grooves are subjected to large shear forces and sometimes to blows, and the screw element must therefore be of a metal capable of resisting such forces. Further, the walls of the threads are subjected to an abrasion resulting from the driving of the material and, in order to increase their resistance to the abrasion, they are usually subjected to a surface treatment, for example by covering them with an anti-abrasion metal. This covering must be renewed periodically, and this operation is costly, especially as the wall of the threads must be machined with precision, for example in machines having two screws which engage each other.